The Bitey Mouthy Puppy
Puppies are like human infants; they explore the world with their mouths. Everything is fair game, so it's up to you to help them learn what they can and cannot chew on. When they choose people, you don't just want to teach them not to put their mouths on you, you want to teach them bite inhibition so if they are startled into swinging around with an open mouth when they are adults the mouth that hits you stays soft and doesn't bite down.
Let's say you're on the floor playing with your puppy. You're holding a toy and she goes from biting at the toy to biting at your hands. Ouch. Puppy teeth are sharp! If she is 8 weeks or younger you can make a squealing sound (like another puppy would do) to let her know that hurt. If she's older the squeal can arouse her more so use a positive interrupter, a happy sound that means "no" without saying no. Most people have a hard time saying no in a happy voice, and you're also likely to say it to people when your puppy is doing nothing wrong, so avoid the no. You can use a happy "uh uh" or "hey hey". The happy voice is important since you want her to have a good association with you, not feel scared. If she backs off, great. Go back to playing with the toy with her. If she goes for your hands again, hand her the toy and remove your hands. You'll do the same thing if she gets more excited. You can still sit there (for now), but she loses your hands playing with her. If she climbs on you to go for your hands again, stand up, cross your arms over your chest to get them out of reach, and don't look at her. (It's often better to cross your arms over your chest rather than just raising your hands so they don't appear as a tantalizing target.) If she puts her mouth on your but does not bite down -- those sharp little teeth aren't hurting you -- just extricate yourself as you replace your hand or arm with a toy. You don't need to correct her, just show her what her mouth should be on.
At this point, if she's still excited, she may go for your feet/shoes or pants. Say "uh uh" and throw a toy away from you. If she runs after the toy or just backs off, get back down on the floor and invite her back to play with the toy. If she resumes trying to attack your feet or pants, say "too bad!" in a cheerful voice and take her to time out. Time out can be a bathroom, laundry room, or large closet with a clear floor and the light on. It should never be her crate. She stays in time out for 5 quiet minutes if she's young, 10 minutes if she's a year a more. If she cries, whines, barks, scratches, etc. the clock resets to zero. When her five minutes are up, let her out with no fanfare and let her go about her business. She can have a toy or chewy in time out. The goal here is for her to reset and also realize that when she doesn't follow the rules she gets separated from her pack.